1. Field of the Invention
The present invention relates to an ultrasound diagnosis apparatus, an ultrasound image processing apparatus, an image processing method, an image display method, and a computer program product.
2. Description of the Related Art
Conventionally, it has been a practice to create a two-dimensional image in which three-dimensional information is reflected, through rendering processing from volume data. For example, according to an ultrasound diagnosis apparatus that creates a three-dimensional ultrasound image by performing a scan with ultrasound waves three-dimensionally (for example, see JP-A 2000-132664 (KOKAI), when a doctor performs diagnostic imaging, a two-dimensional ultrasound image is created from a three-dimensional ultrasound image through rendering processing, and the created two-dimensional ultrasound image is displayed on a monitor.
As a typical method of such rendering processing, a ray tracing method and a volume rendering method can be listed.
According to the ray tracing method, a part corresponding to a surface of a structure is specified on each voxel in volume data, and then an angle of the part specified as the surface of the structure is calculated. A two-dimensional image of a projective plane in which three-dimensional information is reflected is then created by summing brightness values of reflected light that is light emitted from a virtual light source set at a view point reaching the projective plane while reflecting from the surface of the structure specified on each voxel. According to the ray tracing method, an artificial simulation of reflection of light produced on a surface of a structure is executed, so that shades of the structure can be naturally expressed.
According to the volume rendering method, a two-dimensional image of a projective plane in which three-dimensional information is reflected is created by performing multiplication computing of opacity and shading values from a view point continuously in a projecting direction after calculating opacity and a shading value based on each voxel value by considering a state of attenuation and shading of light from the view point when passing through each voxel. According to the volume rendering method, a three-dimensional structure of an object is visually expressed with brightness or variations of color.
Comparing the volume rendering method and the ray tracing method by which it takes some time as a processing time to trace light from a light source, a process procedure can be simplified according to the volume rendering method. For this reason, generally, an ultrasound diagnosis apparatus that creates a three-dimensional ultrasound image uses the volume rendering method as rendering processing to ensure a real-time response when displaying a two-dimensional ultrasound image.
For example, when observing a heart valve with an ultrasound diagnosis apparatus, according to the volume rendering method, contours of the heart valve are displayed in dark color, so that the structure of the heart valve can be visually recognized. Moreover, when performing the volume rendering method, unwanted signals can be removed by limiting a region of interest and executing threshold processing, so that the structure of the heart valve can be more visually recognized.
Specifically, when intending to observe a heart valve, and if a structure (for example, myocardium) that is to be an obstruction is present between the heart valve and a view point; as the obstruction is removed by adjusting a region of interest on which a volume rendering method is to be performed, a doctor can observe the heart valve without the doctor's view being obstructed by the obstruction. Furthermore, according to the volume rendering method, when visibility of a heart valve is decreased due to a low signal that is to be a noise, the visibility of the heart valve for a doctor can be improved by removing the noise through threshold processing.
In this way, a doctor can observe a heart valve without the doctor's view being obstructed by an obstruction or a noise, by optimizing a region of interest and threshold processing when performing the volume rendering. For example, a structure behind a heart valve obstructed by the closed heart valve turns visible in a state that the heart valve is open. However, because a heart valve and a structure behind the heart valve are expressed in similar colors on a two-dimensional ultrasound image created by the volume rendering method; when diagnosing whether regurgitation occurs in the heart of a subject, a region of interest needs to be further adjusted. Specifically, as edges of a region of interest are adjusted to come to heart chambers present in front of and behind the heart valve, there is no structure obstructing a view in front of and behind of the heart valve, so that a background color can be seen in a state that the heart valve is open, and an open-close state of the heart valve can be recognized by determining whether the background color is visible.
In this way, by adjusting a region of interest on which the volume rendering method is to be performed, a doctor can confirm an open-close state of a heart valve by using a two-dimensional ultrasound image created from a three-dimensional ultrasound image by the volume rendering method.
The conventional technologies described above have a problem that it is sometimes difficult to confirm an open-close state of a heart valve in some cases. In other words, even if edges of a region of interest are adjusted to come to heart chambers present in front of and behind the heart valve; when regurgitation is small, contours of the heart valve are displayed in a dark color similarly to a background color. For this reason, according to the volume rendering method used for ensuring a real-time response, it becomes difficult to distinguish between a color originating from a heart chamber and a color originating from contours of the heart valve, among background colors visually recognized between the structures of the heart valve.
Therefore, the present invention has been made for solving the problem of the conventional technologies described above, and an object of the present invention is to provide an ultrasound diagnosis apparatus, an ultrasound image processing apparatus, an image processing method, an image display method, and a computer program product, by which visibility of an open-close state of a heart valve on an ultrasound image can be improved.